Machine sewn seams use a type of stitch called a "lockstitch".  This is how nearly everything in your closet is sewn together.  Although this is the fastest and cheapest mode of sewing, it does not last as long as a properly sewn, hand stitched seam.  When the thread breaks in one place, many stitches will pop in a row, leaving a gaping hole.

I sew all of the leather products with a two needle stitch named for it's origin in sewing saddles, the Saddle Stitch.  When hand sewing with two needles, each stitch gets locked in place with the thread from the other needle.  This is more secure and if a thread breaks over time, the loose thread will be locked in place, and the two pieces of leather will be held together by the other thread.